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Old 05-29-2012, 01:47 PM   #1
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Adding insulation to flex duct


I have R4.2 flex in my hot attic, and it isn't enough. Replacing it all isn't really feasible, and I'm considering wrapping it with some R6 duct wrap if my experiment with one duct and some insulation taken from a leftover flex duct works out. I'm looking at the long rolls of 48" wrap with a taping/stapling flange. About the stapler, I understand I would need an outward clinch stapler. Do these things require you to press firmly against the surface? Obviously that can't work with flex, and I guess the point of a staple gun is to shoot the staples, but it's best to ask, right?

Is the vapor barrier issue any different than wrapping rigid duct?

Any potential issues with a future home inspection?

Any pitfalls with this plan?

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Old 05-29-2012, 02:08 PM   #2
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Adding insulation to flex duct


Make sure the duckwork is sealed up tight.

If you insulate it enough, it should not reach dew point on the surface and have any condensation.

Any pictures?

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Old 05-29-2012, 02:32 PM   #3
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Adding insulation to flex duct


No pics; it's just a typical octopus arrangement with the R 4.2 flex suspended from the rafters with 2" straps. Access to the rear of the house is so bad, it wouldn't be possible to transport and install new ducts and remove the old without crushing much of the cellulose insulation.
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Old 05-29-2012, 03:36 PM   #4
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Adding insulation to flex duct


We usually cut the straps and leave them loose on the floor and blow overtop of them.

Move the cellulose away where the ducts are so that they sit flush on the drywall and you should have enough to cover them at that point.
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Old 05-29-2012, 04:12 PM   #5
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Adding insulation to flex duct


They are routed perpendicular to the 10" joists.
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Old 05-31-2012, 11:31 PM   #6
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Adding insulation to flex duct


Adding insulation adds weight to the flex ducting. At the initial install, the supporting strap spacing (eg.- 5') may have been suitable. You may need closer strapping afterward because if it sags more = a reduction of air-flow because of kinking at the straps;http://www.thefreelibrary.com/Pressu.....-a0201591046
Duct insulation rather than wall cavity insulation is made for them.

I'll move you to HVAC for more responses.

Gary
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Old 06-01-2012, 01:29 AM   #7
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Adding insulation to flex duct


Quote:
Originally Posted by GBR in WA View Post
Adding insulation adds weight to the flex ducting. At the initial install, the supporting strap spacing (eg.- 5') may have been suitable. You may need closer strapping afterward because if it sags more = a reduction of air-flow because of kinking at the straps;http://www.thefreelibrary.com/Pressu.....-a0201591046
Duct insulation rather than wall cavity insulation is made for them.

I'll move you to HVAC for more responses.

Gary
Good point, good info, and thanks for the move.

As an experiment, I wrapped about 85% of one 22' R4.2 duct with some R4.2 and R6 insulation left over from other flex ducts. I'm measuring about 1.5-2 F improvement when the attic is 120+ F, which I expect might improve a little bit with properly installed R6 wrap. That's about a 15% improvement in my split. I'm going to measure it for a few more days then take it all off and measure again before deciding if it's worth it to pursue this. I have another duct of similar length a few feet over that I replaced with a new R6 duct a few years ago I'm also comparing to.

Sweating hasn't been a problem (for the duct, not me) so I guess I could install it a few pieces at a time over a few months if necessary. I've got even longer runs of 40-50' of this horrible flex that would benefit even more. The master bedroom is a degree or two warmer than the rest of the house due to the inadequately insulated flex.

For a number of reasons (access, proximity to other ducts, etc), I may not be able to wrap continuously from start to finish on a given duct. I don't want to do anything that might freak an inspector out, but surely this wouldn't be a problem if the ends are sealed, even though it might look kind of funny in places, right? Considering what did pass inspection, I'm probably worrying a lot more than necessary.

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